A 32-year-old has been arrested on suspicion of terrorism and fraud and 12 addresses are being searched as part of an investigation linked to Syria.
Officers from Scotland Yard's counter-terrorism unit held the man today in west London and he is currently being questioned at a police station in the south of the capital.
He was arrested on suspicion of fraud by false representation and arranging funding for terrorism in Syria. Police said that 12 addresses are being searched - one in Hertfordshire and the rest in west London.
Concerns have been raised for several months about the number of would-be British jihadis who are travelling to the war-torn Middle Eastern state.
Counter-terrorism police launched a campaign earlier this year to encourage women to inform on male relatives who they suspect of planning to travel to Syria to get involved in terrorism.
And last month it emerged that three British men - two from Cardiff and one from Aberdeen - had appeared in a propaganda video for insurgent group Islamic State in Iraq and Greater Syria (ISIS).
Today aspiring jihadi Aseel Muthana told the BBC that he is fighting in Syria and has no intention of returning to the UK.
His brother Nasser appeared with two other British men - 20-year-old Reyaad Khan, from Cardiff, and Abdul Raqib Amin, who grew up in Aberdeen - in an ISIS propaganda video released last week.
A man claiming to be 17-year-old Aseel Muthana told BBC Wales in an internet conversation: "I believe jihad is obligatory, and I never plan to return to the UK."
In another investigation, an 18-year-old, from Grangetown in Cardiff, was arrested over accusations he assisted in the preparation of an act of terrorism.
A spokesman for South Wales Police said: " On Wednesday July 2, officers from North West Counter Terrorism Unit assisted by Wales Extremism and Counter Terrorism Unit arrested an 18-year-old man from the Grangetown area of Cardiff. This arrest is part of an ongoing investigation.
"He has been arrested under the Section 5, Terrorism Act 2006 for assisting in the preparation of an act of terrorism."
South Wales Police later confirmed the 18-year-old man had been released on bail pending further inquiries.